Cosmogenic neutrinos are created when ultra-high-energy cosmic rays ( UHECRs ) interact with extragalactic photon backgrounds .
In general , the expected flux of these cosmogenic neutrinos depends on multiple parameters , describing the sources and propagation of UHECRs .
In our recent paper [ 1 ] , we show that a ‘ sweet spot ‘ occurs at a neutrino energy of E _ { \nu } \sim 1 EeV .
At that energy the flux mainly depends on two parameters , the source evolution and the fraction of protons in UHECRs at Earth for E _ { p } \gtrsim 30 EeV .
Therefore , with current upper limits on the cosmogenic neutrino flux at E _ { \nu } \sim 1 EeV and assuming a certain source class , a constraint on the composition of UHECRs can be obtained .
This constraint is independent of hadronic interaction models and indicates that the combination of a large proton fraction and a strong source evolution is disfavored .
Upcoming neutrino experiments will be able to constrain the fraction of protons in UHECRs even further , and for any realistic model for the evolution of UHECR sources .